28 PRACTICAL ORGANIC CHEMISTRY
Sulphur (Carius). — The process is essentially the same as
that described under the estimation of halogens (p. 22). The
compound is oxidised in a sealed tube with fuming nitric acid,
but without the addition of silver nitrate. The resulting sul-
phuric acid is then precipitated and weighed as barium sulphate.
The same quantities of acid and substance (diphenylthiourea
may be used ; see Prep. 61, p. 159) are taken, and the process of
sealing up and heating, &c., are carried out in precisely the-
same way as for the halogens. The contents of the tube, after
heating, are cautiously diluted with water and then washed out
into a beaker, and filtered, if necessary, from fragments of glass.
The filter paper is then well washed with hot water and the
filtrate diluted to at least 250 c.c. with water. The liquid is
heated to boiling, and a few c.c. of barium chloride solution
added. On continued heating over a small flame the liquid
clears and the precipitate subsides. The addition of another
drop of barium chloride will determine if the precipitation is
complete. The liquid is then filtered through an ordinary
funnel, the precipitate of barium sulphate washed with hot
water, dried and weighed in the usual way.
Example. — Diphenylthiourea gave the following result : —
0*2518 gram gave 0*2638 gram BaSO4.
= 14*39 per cent.
Calculated for C13H12N3S ; 8 = 14-05.
Determination of Molecular Weight
According to Avogadro's law, equal volumes of all gases
under similar conditions contain the same number of molecules.
Consequently the weights of equal volumes or the densities of
gases will represent the ratio of their molecular weights. If the
densities are compared with hydrogen as the- unit, the ratio
in which W8 and Wh are the weights of equal volumes of
substance anji hydrogen respectively, will give the molecular
weight of the substance compared with the molecule or two
atoms of hydrogen or half the molecular weight compared with